The present invention relates to subterranean well cementing operations, and more particularly, to cement compositions comprising high aspect ratio materials and methods for using such cement compositions in subterranean formations.
Hydraulic cement compositions commonly are utilized in subterranean operations, particularly subterranean well completion and remedial operations. For example, hydraulic cement compositions are used in primary cementing operations whereby pipe strings, such as casing and liners, are cemented in well bores. In performing primary cementing, a hydraulic cement composition is pumped into an annular space between the walls of a well bore and the exterior surface of the pipe string disposed therein. The cement composition sets in the annular space, thereby forming therein an annular sheath of hardened, substantially impermeable cement that supports and positions the pipe string in the well bore and bonds the exterior surface of the pipe string to the walls of the well bore. Hydraulic cement compositions also are used in remedial cementing operations, such as plugging highly permeable zones or fractures in well bores, plugging cracks and holes in pipe strings, and the like.
When the cement compositions contact permeable subterranean formations, fluid (e.g., water) may be lost into the formation. Excessive fluid loss may cause the cement composition to become prematurely dehydrated, thereby potentially causing bridging in the annulus and limiting the time for which said slurry can remain pumpable, and/or reducing bond strength between the set cement composition and a subterranean zone, the walls of pipe and/or the walls of the well bore. Fluid loss control additives (e.g., polymers and copolymers) may be included in a cement composition, inter alia, to reduce fluid loss into the formation. When the permeability of the formation is high, for example, because of unconsolidated or depleted formations, or microfractures, the fluid loss control additives (e.g., polymer and copolymers) may not be effective in preventing fluid loss from cement compositions. In these instances, lost circulation materials have also been included in cement compositions, inter alia, to reduce fluid loss into the formation. Examples of loss circulation materials that may be included in a cement composition include peanut shells, mica, cellophane, walnut shells, calcium carbonate, plant fibers, cottonseed hulls, ground rubber, and polymeric materials.
Another problem encountered in subterranean operations may be failure of the cement sheath. Once set, the cement sheath may be subjected to a variety of shear, tensile, impact, flexural, and compressive stresses that may lead to failure of the cement sheath, resulting in, inter alia, fractures, cracks, and/or debonding of the cement sheath from the pipe string and/or the formation. This can lead to undesirable consequences including, inter alia, lost production, environmental pollution, hazardous rig operations resulting from unexpected fluid flow from the formation caused by the loss of zonal isolation, and/or hazardous production operations. Cement failures may be particularly problematic in high temperature wells, where fluids injected into the wells or produced from the wells by way of the well bore may cause the temperature of any fluids trapped within the annulus to increase. Furthermore, high fluid pressures and/or temperatures inside the pipe string may cause additional problems during testing, perforation, fluid injection, and/or fluid production. If the pressure and/or temperature inside the pipe string increases, the pipe may expand and stress the surrounding cement sheath. This may cause the cement sheath to crack, or the bond between the outside surface of the pipe string and the cement sheath to fail, thereby breaking the hydraulic seal between the two. Furthermore, high temperature differentials created during production or injection of high temperature fluids through the well bore may cause fluids trapped in the cement sheath to thermally expand, causing high pressures within the sheath itself. Additionally, failure of the cement sheath also may be caused by, inter alia, forces exerted by shifts in subterranean formations surrounding the well bore, cement erosion, and repeated impacts from the drill bit and the drill pipe.